The purpose of this work is to study the mechanism of idiopathic and transfusion-associated clearance of neutropenias by applying an opsonic assay of circulating antineutrophil antibodies. Serum is tested by antibody activity by reacting it with normal granulocytes and then incubating the reacted granulocytes with other normal (responder) granulocytes. If antibody is present, the responder cells will increase their rates of 14C-l-glucose oxidation.